How roof area is calculated
A sloped roof has more surface than the ground it covers. Multiply the footprint (including overhangs) by the pitch factor:
Roof area = length × width × √(1 + (pitch ÷ 12)²)
A 40 × 30 ft footprint with a 6/12 pitch: 1,200 × 1.118 = 1,342 sq ft. With 10% waste that's 14.8 squares, or 45 bundles of shingles.
Pitch multipliers
| Pitch | Angle | Multiply by |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.250 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
Squares and bundles
Roofers measure in squares: one square is 100 sq ft. Most architectural asphalt shingles come 3 bundles to a square. Add 10% waste for a simple gable roof and 15% or more for hips, valleys and dormers, plus starter strips and ridge caps.
This gives a material estimate for a simple roof. For a quote, a roofer will measure the actual roof, including every hip, valley and penetration.